1. Field of the Invention
The present invention relates to improvements in pulse-compression systems for laser telemetry and Doppler measurement which are primarily intended to equip fire control systems.
More specifically, the invention is concerned with a system of this type in which means are provided for acquisition and tracking of the Doppler frequency shift generated by the radial velocity component of the moving target that is being tracked.
2. Description of the Prior Art
In the case of a radar system, the Doppler frequency shift hardly produces any disturbances, taking into account the wavelength values of the radar system and the medium-frequency passband of the receiving circuits.
On the other hand, in the case of laser telemeters or lidar systems, the wavelength is very short and the Doppler frequency which is given by the expression F.sub.D =2V.sub.R /.lambda. becomes very high in respect of small variations in the radial velocity V.sub.R. By way of example, in the case of a lidar having a wavelength .lambda.=10.59 .mu.m, the Doppler frequency shift varies by 0.19 MHz each time the velocity varies by one meter per second. In consequence, the limits of the medium-frequency passband of the receiver are reached in a very short time. For example in the case of a passband of .+-.12.5 MHz on each side of an intermediate center frequency of 150 MHz, these limits are reached in respect of a radial velocity of the order of .+-.240 km/hr. This results in narrow operating limits beyond which there is a loss of information of the signal which is no longer processed by the receiver. In consequence, the system presents difficulties arising from its basic design concept. One solution would be to increase the bandwidth downstream of the photomixer with all the disadvantages attached to this solution.
French patent Application No. 79 19 970 of Aug. 3rd, 1979 granted under No. FR-A-2 462 717 disclosed a system for laser telemetry and Doppler measurement with pulse compression, which permits measurement of both distance and radial velocity and is more particularly adapted to unambiguous discrimination of a plurality of detected targets. In this system, provision is made at the transmitter for two pairs of delay lines in which the time delays are variable as a function of the frequency and which have slopes +K and -K, +K' and -K' respectively, the same set of lines being employed in conjugate relation at the receiver. By means of these delay lines, two transmission pulses can be produced periodically by employing a different pair of lines from one period to the next and a reliable measurement of the distance and radial velocity of each detected target can be deduced from the instants of reception of echos by means of a simple calculation with removal of ambiguity.
In practice, however, this solution is applicable only to targets which are traveling at a relatively moderate speed.
By reason of the dispersive lines, the process employed is in fact suitable only within a range limited to approximately .+-.2 MHz. This corresponds to radial velocities which do not exceed .+-.10 m/sec, that is to say velocities in the vicinity of .+-.36 km/hr. This system is therefore more particularly applicable to surface-to-surface systems and not to surface-to-air or air-to-air systems in which high-speed targets may travel at speeds of the order of several Mach numbers.
Moreover, French patent Application No. 82 00238, published as French Pat. No. 2 519 771 relates to a pulse-compression lidar which is equipped with means for acquisition and tracking of moving targets and retains the characteristics of matching and sensitivity of the receiver. These means involve frequency transposition of a local oscillation laser wave and locking of the transposed local frequency when beating by superheterodyne mixing with the reception light signals produces an electric signal at the intermediate frequency F.sub.I. After acquisition, the transposition frequency F.sub.T is controlled automatically in dependence on the Doppler shift. The target acquisition and tracking means comprise an acoustooptical delay-line device controlled by sawtooth signals via a frequency synthesizer. At the time of acquisition, the receiver initiates locking of the local beat frequency F.sub.L +F.sub.T, then produces automatic adjustment of the transposition frequency F.sub.T to the instantaneous value of F.sub.D (Doppler tracking).
The major drawback of this solution lies in the length of time of the search and acquisition stage during which a frequency excursion is carried out by scanning of the synthesizer. This frequency excursion in fact makes it possible to cover the entire processing range of Doppler frequencies and the search time is therefore related to the location of the Doppler value to be found within this range. This search time is longer as coincidence takes place nearer the end of the frequency excursion.